The use of steel cords to reinforce belts was probably first introduced to reinforce conveyor belts. Such belts basically consist of a set of steel cords encased in a rubber matrix. A steel cord conveyor belt runs on idlers and is tensioned between a receiving and discharge station for transporting materials. Evidently, the size of the steel cord for reinforcement of conveyor belts is relatively large—with diameters from 2 to 14 mm—as the acting forces are large. At the other end of the spectrum precision timing belts emerged reinforced with very fine steel cords, assembled out of steel filaments with a diameter of 40 μm to 150 μm but now encased in thermoplastic polymers in stead of rubber.
Recently, steel cords enjoy renewed attention for reinforcement of strips that are used in relatively static applications such as to reinforce pipes or hoses. See for example FR 2 914 040, EP 1 760 380, WO 2002/090812. But the use of flexible strips can be even more diverse in that it can also be used as a retrofit material to repair damaged concrete structures or to protect mooring lines from cutting.
While the word ‘belt’ in a technical context is mainly used in dynamic situations where movement or power is transferred, the word ‘strip’ seems to be more used in a static context to transfer force without movement. In what follows only the word ‘strip’ will be used, although it should be clear that for the purpose of this application, this may equally well refer to a ‘belt’.
There has always been a need to connect the ends of a strip to one another in order to make it endless, or there was the need to joint several single pieces of strip together to make a very long strip. Such a connection is referred to as a ‘joint’ or a ‘splice’ which are considered synonyms for the purpose of this application. An ideal splice should not be noticeable in the strip. The splice must therefore have:
(A). Equal breaking strength as the strip;
(B). Equal stiffness in stretching as well as in bending as the strip itself;
(C). Equal dimensions as the strip (no thicker sections)
(D). Show equal dynamic fatigue as the strip;
(E). Should be relatively easy to implement on site.
In principle an ideal splice could be made when the length of the splice is unlimited. However, this is not practical.
Hence, making practical splices is always compromising between the different requirements (A) to (E) mentioned above. One type of splice may therefore be perfectly fit in one application, but not for connecting another kind of strips in another application. The following splice methods are known:
Splice by means of mechanical fasteners: a row of clamps are attached to the edge of the belt that is cut perpendicular to its length. A connecting rod is introduced into the inter-digitised eyelets formed. This kind of splice is used for fabric reinforced types of strips. It can not be used for strips with mainly axial reinforcement (the clamps tear out). As the splice is like a hinge, it is more flexible than the strip itself.
Overlap splice: the ends of the strips are overlapped and are vulcanized or glued to one another. This kind of splice is sometimes used to make rubber tracks endless. It has an increased bending stiffness in the splicing area because the two cord planes of both ends do not coincide and form a stiff double layer.
Interlocking splice: the ends of the strips are cut in the plane of the strip according a pattern with protrusions and recesses that fit into one another (like a dovetail connection). Afterwards the splice is vulcanized or glued or molten together. See for example WO 2009/040628.
The following splices are described in the ISO 15236-4 standard.
Finger splices are splices in which the ends of the strips are cut into a matching saw tooth pattern. The ‘fingers’ are afterwards vulcanized to one another. It is mainly used for fabric reinforced belts or strips, hence is less appropriate if only longitudinal oriented steel cords are present.
In an ‘Interlaced stepped splice’ cord ends from one strip are arranged—‘interlaced’—between cords of the other strip end and subsequently covered with rubber or polymer or glued together. ‘Cord end’ is to be regarded as that part of the cord that is in the splice. The place where the cord stops is called the ‘butt end’. The butt ends of the cords usually finish at regular positions in the splice hence its name of ‘stepped splice’. An ‘interlaced stepped splice’ can thus be defined as a splice wherein the number of cords in the splice area is always larger than the number of cords in the strip. Such a splice has a very good static strength (the splice can be stronger than the belt) but shows an increased stiffness in the splice area. Stepped interlaced splices can be used if the strip has less than about 50% linear packing density. With ‘linear packing density’ is meant the ratio of the sum of all cord diameters to the total width of the strip. In case of a larger linear packing density, either some cords will have to be cut at the splice entry from both belts in order to accommodate space for the inserted cords leading to a loss of strength or the splice area can be made broader than the strip in order to accommodate the increased number of cords.
A ‘plain stepped splice’ can be defined as a splice wherein the number of cords within the splice remains equal to or smaller than the number of cords in the strip. In other words: the cord butt ends of both strips abut to one another. Different lay-out patterns are possible such as an ‘organ pipe splice’ (having a repeating 01230123 . . . pattern, the numbers indicating the step length of the cord ends of one strip in the splice) or a ‘fir tree splice (having a repeating 01232100123210 . . . ) pattern. Such splices are difficult to discern from the strip itself in terms of bending stiffness, axial stiffness and section. However, they show a lower strength compared to ‘Interlaced stepped splices’.
The inventors were primarily concerned with finding good splices of the ‘interlaced or plain stepped splices’ for strips reinforced with steel cords and encased in a thermoplastic polymer material. Although there is an extensive literature on how such splices are to be constructed for rubber steel cord reinforced belts, little exists for thermoplastic steel cord reinforced strips. For the former, a good overview can be found in “Design of Steel Cord Conveyor Belt Splices” by M. Hager and H. von der Wroge in “Belt Conveyor Technology, I/94” out of “The Best of Bulk Solids Handling 1986-1991” published by Trans Tech Publications.
Basically, there are two aspects to a stepped splice:
There is the mechanical aspect wherein the lay-out scheme of the splice plays a mayor role. Indeed, by seeking different interlacing patterns (for ‘interlaced stepped splices’) or abutment arrangements (in ‘plain stepped splices) an improvement in splice performance can be obtained.
On the other hand it will be clear that in the kind of splices envisaged, no direct connection is made between steel cords from the one strip to the steel cords of the other strip. Hence all force has to be transferred from the cords in the one strip to the cords of the other strip by mediation of the polymer that is in contact with the steel cords. Ergo two factors will be crucial in this transfer: the strength of the anchorage of the steel cord to the polymer and the strength of the polymer.
Although both aspects have a high interaction with one another and can not be fully separated from one another, the focus of this application is on the second, chemical aspect and more in particular how an optimal chemical bonding can be implemented in a splice.
An inventive lay out scheme is the main subject of a co-pending application of the same applicant filed on the same day as this application and covers the splice from the more mechanical viewpoint.